This is some information I discovered on aligning the speedometer and setting the odometer on a 1989 Audi 100.  It will probably also be useful to owners of BMWs, Volvos and Vauxhalls where similar circuits are used.

In 1989 (or thereabouts), Audi changed from mechanical speedometers to electronic.  A sensor (variable reluctance?) is bolted into the gearbox, presumably on the final drive.  The speedometer appears to be made by VDO and uses a moving coil meter movement for the speedometer and a stepper motor to drive the odometer.

I had a go at reverse engineering the electronic circuit and came up with the following:

  Audi 100 Electronic Speedometer (.cdr file)
  Audi 100 Electronic Speedometer (.gif file - large)

If you need to increase the miles displayed on an odometer (to get a replacement to show the true mileage of a car), connect the speedometer up to a 12V PSU and an audio oscillator set to +12dBm, 600ohms, 315Hz. The dividing ratio of the speedometer is 6644 (printed on the bit of paper stuck to the back, so 315Hz corresponds to 170.68mph).

I noticed that the speedometer wasn't very accurate. 30, 60 and 90mph should correspond to 55.4, 110.7 and 166.1Hz. My speedometer was close at 30mph, about 5mph out at 60mph and more than 10mph out at 120mph (not that I plan on doing much of this speed).

The chip that drives the speedometer and the odometer is an ITT UAF2115. Originally, I couldn't find any reference to this chip anywhere, so I e-mailed ITT. They sent me a PDF data sheet for a UAF1025 which apparently is identical.  More recently, somebody managed to locate the PDF data sheet for the UAF2115 and emailed it to me - thanks Paul.  These data sheets show that there is a series resistor connected to the speedometer which is used to adjust the speedometer sensitivity. The resistor on the Audi circuit board was 51ohms. I found that 54ohms gave a much better response along with a realignment of the mechanical zero position. The speedometer is now within 1mph between 30 mph and 60 mph and within 2mph over the rest of the useful range.

Of course, the speedometer is only accurate when the tyres have the correct diameter, but the anal retentive amongst us could use a variable resistor to compensate for tyre wear (just kidding). 

Note: These adjustments have no effect on the odometer. That just divides the sensor pulses to give 1mile for every 6644 pulses.

The most common reason for failure of these devices is bad connections and dry joints.  Dismantle the dash and clean/spray all the connectors.  Check all solder joints inside the dash and on the pcb, and if in doubt, resolder them.  A couple of people have emailed me to report that the 220uF cap across the meter had gone short.

Good luck!